Balanced duplexer



2 SHEETS-SHEET 1 Filed July 50, 1948 /Nvsfv'rolz HENRY J. R/BLET Bv E i Arn/Ev Filed July 30, 1948 2 SHEETS--SHEET 2 HENRY J. R/BLE T Patented Feb. 26, 1.952

BALANCED DUPLEXER Henry J. Riblet, Belmont, Mass., assignor to Raytheon Manufacturing Company, a corporation of Delaware lApplication July 30, 1948, Serial No..41,509

19 Claims.

This invention relates to duplexers for radar systems of the type that employ a. single antenna for transmitting and receiving, and more particularly to an improved balanced duplexer for such systems.

It is an object of the invention to provide a balanced duplexer which affords greater broadband characteristics than have heretofore been available.

Itis another object to provide a duplexer which aifords optimum decoupling between the transmitter and receiver during reception, and improved protection to the elements of the receiver input circuit during transmission.

It is another object: to provide a balanced duplexer which is more compact, smaller, and less complex than known balanced duplexers employing magic tee and rat race bridges, and which is adaptable to construction in small sizes comparable -with known single-tube duplexers.

The basic idea of a balanced duplexer is well known to the art. In an article by E. G. Schneider entitled Radar, Proceedings of the LR. E., August 1946, starting at page 551, mention is made of two specific types, one involvingy a ratrace type of circuit, and the other using` magic tees. Such balanced duplexers are discussedl in Microwave Duplexers, by L. D. Smullin and C. G. Montgomery, McGraw-Hill Book Co., Inc., New York, 1948; pages 350 to 369, inclusive. The rat-race bridge is called a ring-circuit magic tee in this book.` These two types ofA balanced duplexers are. characterized mechanically by space-consuming angular waveguide or coaxial line junctions, and electrically by line sections' of critical len'gth, such as' quarter wave sections, which are necessary to the proper operationy thereof. The balanced duplexer of the present invention employs directional couplers in'place of rat-race bridges and magic tees, with a consequent economy of space and reduction in complexity. No line section of critical length is required. Y

In general, balanced duplexers have certain advantages over the single-tube types, among these being the fact that they have better broadband characteristics. The present duplexer has these advantages in addition to theparticular advantages thatl how from its unique construction. Other and further advantages and features ofthe invention will become apparent from the description thereof that follows, reference being made to the accompanying drawings, wherein:

Fig. 1 is a side sectional View of an embodiment of the invention;

Fig. 2 is a fractional section along line 2-2 of Fig. 1, showing a planA view of a directional coupler;A

2 Fig. 3 is a cross-section along line 3--3 of Fig. 1;

Figs. 4A and 4B are diagrams illustratingfthe operation of the directional couplers; and

Figs. 5A and 5B and 6A and 6B are vectorl diagrams illustrating the operation of the directional couplers.

Referring now to Figs. 1, 2 and 3, two similarl rectangular waveguides II and I2 are directionally coupled together by a directional coupler A through a common wide wall I 3. The coupler has two input ends I4' and I5, and two output ends It and Il. The input ends I4 and I5l are connected tothe antenna andl the transmitter, respectively (both not shown), of the radar system, and the output ends I6 and I1 are eachcormected to an electronicswitch. device, here is a pairv ofr T-R tubes I8 and IS, respectively.

The T-R tube is an impedance changing device which functions in arwell-known manner to` provide in eiect a short circuit when it is` fired by a voltage exceeding the firing value. The present T-R tubes I8 and' I9- each provide such a short circuit across the transmission line in which it connected when the transmitted voltage exceeds the firing value, butv is effectively a matchedv section of theline at lower voltages. These tubes are constructed in a section of rectangular waveguide of the same kind electrically as that used for transmission line in the radar system,. and" are each provided with a wave-pervious gasinpervious window 2'I at each end and a pair ofl spark electrodes 22A and 2'3 disposed in the' guide section between the windows in the region of highest voltage'gradient for the fundamental or TE0,1 mode. Oneelectrode 23 in ea'ch` tube is formed' as a hollow truncated cone and'h'as-y aJ keep-alive electrode' 27 disposed within'it. The

keep-alive electrode is insulated from and 'sup ported on the metal of the waveguide-by anon-- conductive, gas impervious support 24, which may be madeofglass', andprojects therethrough; terminating in'an electrical connectionY capl. A keep-alive potential is appliedl tol each 'ca-p 25 by a suitable unidirectional Voltage source, suchas a battery 26. As-willbe shown, thefpre'sent invention is' inherently ai bro'adi-ba'nd` device. To take full advantage of the broad-bandipropy erties of the invention, it is preferred tol em.- ploy a T-R tube which also is broad-band in nature, for example, thev multi-gap type 1B63, which is described in the aforementioned boolamullinv and Montgomery, at page 231; and in1theory at page 111. It is understood, however, thatfother types of T-R tubes may be used if desired.'

A second pair of mutually similar rectangular waveguards 3l and 32 are directionally coupled together through a commonwide wall- 33 bya second directional coupler B. Coupler B has two input ends 34 and 35 and two output ends 36 and 3l. The input ends 34 and 35 oi coupler B are connected to the T-R tubes I8 and I9, respectively, and the output ends thereof are connected to a non-reflective termination 38 and to the radar receiver lnot shown), respectively. The non-reflective termination may conveniently be a wedge-shaped piece of hard wood, or Bakelite, coated with an electrically resistive material, such as colloidal carbon, known as Aquadag.

It will be seen that the balanced duplexer of Fig. l comprises essentially two separate waveguide transmission lines,one consisting of sections II, I8 and 3I connected between the antenna and the non-reiiective ter'nination 38, and the other consisting of sections I2, I9 and 32 connected between the transmitter and the receiver, the lines being directionally coupled together by couplers A and B at two spaced apart points or regions, and each having a set of T-R tube electrodes 22 and 23 which are effective at a point as region intermediate the coupling points or regions. The duplexer has as its inpur, terminals the input ends I4 and I5 of coupler A, and as its output terminals the output ends 36 and 3l of coupler B.

The directional couplers A and B are preferably of any type which can be dimensioned to function as a hybrid; thatJ is, which can be so dimensioned that power that is incident upon a terminal at one end is evenly divided and appears in equal quantities at both terminals at the other end. Such a coupler is said to have a coupling ratio of lzl, or unity. Inasmuch as the power which is incident upon a terminal at one end of the coupler appears at the other end thereof, each directional coupler A or B may be said to be of the kind which couples in the forward direction.

In addition, the couplers should preferably be electrically symmetrical; that is, a wave being coupled should encounter the same electrical conditions on either side of it. Such a coupler is a quadrature phase shift device, and its employment simplifies the construction of a broad-band balanced duplexer, as will be explained. It will be helpful in understanding the operation of the invention to understand that quadrature phase shift is characteristic of symmetrical directional couplers in general and of the couplers illustrated in particular, as will be explained below. The couplers A and B that are illustrated in Figs. 1 to 3, inclusive, are preferred for their inherently small size, natural symmetry, and ease of construction and design.

Couplers A and B each consist of a plurality of transversely directed slots 42 centered substantially along the center line of the wall I3 or 33, respectively, in which they are cut, and a plurality of longitudinally directed slots 43 disposed near the edges of the wall. The couplers are of the kind which is described in detail and claimed in copending application, Serial No. 784,277, filed November 5, 1947. It will be recognized by those skilled in the art that when the fundamental or TEo,1 mode of wave propagation exists in the various waveguides, the individual slots 42 and 43 of the couplers are each disposed perpendicular to the direction of current flow in the wall in which it is cut, so that each slot is disposed advantageously to be excited by this mode. The slots are preferably all made thin, so that each slot will be excited principally by that field which 4 gives rise to a current tending to ow across its long dimension, and no other.

The coupling ratio of each coupler is adjusted to unity, so that each coupler functions as a hybrid, and couples substantially one-halfof the power presented to it. To this end the slots of each coupler are made non-resonant to waves in the operating frequency band, so that each slot couples only a small amount of the power presented to it, and the number of slots of each coupler is then chosen to effect a coupling ratio of unity. In order that the coupling ratio shall remain reasonably constant over the operating frequency band, all the slots are maintained short with respect to the longest guide wavelength being coupled; the slot lengths should not be equal to or exceed one half the longest guide wavelength in the band. With respect to the present couplers, these features of construction are explained in detail inthe aforementioned copending application, where the form of coupler in which the coupling ratio is unity is referred to as a bridge circuit. As developed in said copending application, the spacing between slots of couplers like A and B has little or no eiect on the coupling ratio, but there is an improvement in directivity when the spacing is reduced, as in the embodiment illustrated in Fig. ll of said application. The coupling ratio is determined by the nature of the individual slots and the number of slots employed in each coupler.

Each of the directional couplers A and B is a quadrature phase-shift device; that is, the output voltages differ in phase by degrees. Their quadrature nature is due mainly to their symmetry; for example, a coupler A couples two identical waveguides II and I2 symmetrically; that is, the same mechanical and electrical comguration is presented to waves on either side of the coupler.

Considering now one of the symmetrical directional couplers, for example, the coupler A, if the coupler were innitely directive in the forward direction, an ideal situation which is not fully realized but is closely approached in practice, all the power incident upon either of its input ends I4 or I5 would become available at the two output ends I6 and Il in equal amounts, because the coupler is a hybrid. The ideal situation can be synthesized for symmetrical directional couplers in general by introducing two separate sets of waves into the two input ends, as illustrated in Figs. 4A and 4B, where coupler A is considered as an example in which power is incident upon input. end I4. As shown in Fig. 4A, two waves of equal intensities and symmetrical, or of like phase as seen from the common wall I3, represented by vectors 5I and 53, are assumed to be introduced into or incident upon the two input ends I4 and I5, respectively. Simultaneously, as shown in Fig. 4B, two additional waves which are antisymmetric or mutually oppositely phased as seen from the common wall I3, represented i, by vectors 52 and 54, are assumed to be incident upon the input ends I4 and I5, respectively. Vector 52 is in phase with Vector 5I, in consequence of which vector 54 is oppositely phased to vector 53. Thus, at one input end I4, a voltage exists, which is the sum of two symmetric.

voltages, represented by vectors 5I and 52, while the voltage at the other input end I5 is zero, being the sum of two antisymmetric vectors 53 and 54 of equal magnitude. This is the actual physical situation that exists at the input ends of the directional coupler when power is introduced through input end I4, and the directivity is ideal, or innite.-

Consider now the condition of the four vectors 5I, 52, 53 and 54 at the output ends I6 and II of the coupler. Assuming no reections, which is the case for ideal performance, i. e. infinite directivity, and perfect matching, all the incident power will arrive at these output terminals in equal amounts, except for Very small line attenuations aiecting all the vectors equally, it being remembered that the transmission paths are similar. However, at the coupler A, where the common wall I3 is slotted and the fields in the two waveguides can interact,` vectors 5Il and 53 generate, in effect, a symmetrical mode of transmission in the two coupled waveguides II and I2, while vectors 52 and 54 generate, in effect,

a different, antisymmetric mode. In general, these two coupling modes have different phase velocities in the waveguides at the coupler. It has been found that the phase Velocity of the antisymmetric mode is not substantially changed from the normal waveguide phase velocity of the individual components, but that the phase velocity of the symmetric mode tends toward the lower free space velocity of electromagnetic waves. Thus, while vectors 5I and 53 maintain their initial symmetry, and vectors 52 and 54 maintain their initial antisymmetry, the phase relation between vectors of one coupling mode and those of the other coupling mode is different at the output ends I6 and I'i from the relation that exists at the input ends I4 and I5. This situation is illustrated in Figs. 5A and 5B, where vectors 5I and 53 are still symmetric and vectors 52 and 54 are still antisymmetric, but vectors 52 s and 54 have been shifted clockwise, each by the same angular amount, with respect to vectors 5I and 53, respectively. The amount that the Vectors of one mode are shifted with respect to the vectors of the other mode depends on the length of the coupler. The vectors 5I and 52 in the rst output end I'B (Fig. 5A) have a resultant phase illustrated by vector 55, while the vectors 53 and 54 in the second output end I'I (Fig. 5B) have a resultant phase illustrated by vector 56. The phase difference between the two resultants is 90 degrees, and since the foregoing discussion is general to symmetrical directional couplers, this is true regardless of the kind of symmetrical directional coupler that i`s used, or the frequency of operation.

The general symmetrical directional coupler is thus seen to be a quadrature phase shift device for all frequencies and regardless of the coupling ratio; that is, at the output terminals the source or direct voltage and the coupled voltage differ in phase by, 90 degrees; or in other words, the coupler introduces a IBO-degree phase shift in the coupled voltage. It is thus distinguished from the magic tee, which is characterized by a phase shift of 180 degrees or Zero. Which of the output voltages, as represented by vectors 55 and 56, lags or leads the other is determined by the nature and length of the particular directional coupler employed, but in a particular coupler the sense of the phase difference is the same regardless of the direction in which power passes through the coupler, since in both directions the paths are symmetrical. In the present directional couplers, where the coupling is through a common wide wall I3 or 33, the coupled voltage 56 vlags Vbehind the source voltage 55.

Since the directional couplers that are employed in the invention arey also hybrids, the

arrived at by proper adjustment of the length of the coupler as discussed above, is illustrated. The length of the coupler is such that vectors 5I and 52, and 53 and 54, are respectively 90 degrees apart, in consequence of which the resultants 5'! and 58 are equal, but still in phase quadrature. The physical significance of this is that while the quadrature phase shift character of a symmetrical directional coupler is fixed regardless of the coupling ratio, the electrical length for a symmetrical hybrid directional coupler is 9G degrees different for one coupling mode than for the other; i. e., the length is such that in each output end the voltage of one mode is 90 degrees different in phase from the voltage of the other mode. The present coupler is aptly termed a quadrature hybrid. The foregoing discussion applies equally to directional coupler B, which is also symmetrical, so that each coupler is a quadrature hybrid.

The operation of the dupleXer of Fig. 1 is as follows. When the transmitter is operated, power enters the transmitter input terminal I5, and is divided equally in directional coupler A, equal portions arriving at the T-R tubes I8 and I9, but with the voltage at tube I8 lagging that at the other tube by 90 degrees. This power fires the two T-R tubes simultaneously, and is reflected into coupler A. Power reflected from the upper T-R tube I8 divides equally at the coupler and proceeds into the input terminals I4 and I5 in equal amounts, with an additional phase lag of 90 degrees, or a total of 180 degrees, in the voltage reflected into the transmitter input terminal I5. Power reflected from the lower T-R tube I5 di vides equally at the coupler and proceeds into the input terminals I4 and I5 in equal amounts, with no voltage phase change introduced into the power reected to the transmitter terminal I5 and a voltage phase lag of 90 degrees introduced in the voltage reflected to the antenna terminal I4. Thus, the voltages of the two reflected power components in the transmitter input terminal I5 are antisymmetric, or 180 degrees out of phase with each other, while the voltages of the' two reiiected power components in the antenna terminal I4 are symmetric, or in the same phase,

since each component' has passed through the coupler A once, and has been delayed by the same amount, namely degrees. Since the two power components at the two T-R tubes are equal, there is substantially complete cancellation of reflected power at the transmitter input terminal I5. The reflected power components in the antenna terminal I4 are added to each other and proceed to the antenna.

In general, the major portion of the power that leaks or escapes through the T-R tubes and arrives at the receiver during transmission is that which is generated in the tubes themselves, such as noise. Measurements made on the present balanced duplexer have indicated that there is very little component of leakage power that is related in phase to the transmitter power. In this respect, the present duplexer is comparable to other known balanced duplexers, and superior to the conventional single tube duplexers, with or without the anti T-R. Components of leakage power that are in phase with the transmitter power can be" expected to be generally proportional thereto in magnitude. No such component has been detected with the present duplexer. As a consequence, the protection afforded during transmission by the present duplexer to the crystals employed for radio frequency detection in microwave radar receiving systems is excellent. Except for the losses due to the T-R tubes themselves and normal line losses, all the power from the transmitter proceeds to the antenna. The voltage Vo that is introduced into the duplexer from the transmitter is divided vectorially in coupler A, each portion having the magnitude:

1/5 Each portion is further divided after reiiection from the T-R tubes, the ultimate portions each having the magnitude:

these ultimate portions, the voltage present in the antenna terminal I4 is whereas the voltage present in the transmitter input terminal I5 due to reflection from the T-R. tubes I8 and I9 is Thus, the total voltage of the source is applied to the antenna, and hence the total power.

During reception, the power that is incident upon the antenna is brought to the antenna terminal I4, and thence to coupler A, Where it is divided evenly and proceeds in equal amounts through the two T-R tubes I8 and I9, without ring the tubes. The two waveguide sections of the tubes are matched to the respective output ends I5 and Il of coupler A and the respective input ends 34 and 35 of coupler B to which they are connected. The voltages in the T-R tubes have a phase diierence of 90 degrees, with a particular sense, which in the present embodiment is such that the voltage in the lower tube I9 lags. The power from the upper tube I8 enters coupler Bf and is divided evenly, equal portions proceeding into the duplexer output terminals 3B and 3l, with a phase diierence of 90 degrees. Since coupler B is identical to coupler A, the sense of this phase difference is the same as that introduced by coupler A in operating on the antenna power. Thus the voltage in the receiver terminal 3l from the upper T-R tube I8 is delayed by 90 degrees in passing through coupler B and lags that in the closed terminal 36 from the same tube I8 by 90 degrees. Since the voltage in the lower tube I9 already lags the voltage in the upper tube I8 by 90 degrees, the two voltage components from the two T-R tubes are in the same phase in the receiver terminal 3l, and are added to each other. It may be noted that each voltage component in the receiver terminal 3l passed through one coupler in arriving there from the antenna. The voltage from the lower tube I9 that reaches the closed terminal 35 through coupler B must pass through two couplers in arriving there from the antenna so that it is delayed by a total of twice 90 degrees with re- Vo Voz() spect to the voltage that arrives in that terminal T5 Recovery time a property of the via the upper tube I8 so that, being of equal magnitude, Vthese two voltages cancel each other. The non-reflective termination serves to absorb and prevent the reflection of any small uncancelled voltage component that may be present in the closed terminal 36. In the same manner as was explained above with relation to transmitted power, all the power received at the antenna is furnished to the receiver, except for general system losses, which are small and not due to the invention.

The value of employing quadrature phase shift directional couplers is now seen to be that with them it is only necessary to place both T-R tubes I8 and I9 the same effective distance from each coupler A or B to attain a broad-band balanced duplexer; that is, the eiective short circuit of each tube is the same distance from coupler A for each tube, and the same distance from coupler B for each tube. The distance from A is not necessarily the same as the distance from B, however. The proper dimensions are arrived at simply by constructing a symmetrical system, as shown in Fig. l. Then, automatically, substantially all the transmitted power introduced at terminal I5 goes to the antenna via terminal I4, and substantially all the received power at terminal I4 goes to the receiver terminal 31. No quarter wave or other phase adjusting section is necessary with this construction to render the device frequency critical.

In the embodiment of the invention described above, the component parts are all available with broad-band characteristics. The couplers A and B can be made broad-band as taught in the aforementioned copending application. The T-R tubes are available with broad-band characteristics, for example, the type 1B63. Data taken from a model designed for operation at the frequency 9090 mc./sec., employing broad-band components, is as follows:

I. The low level data, namely the performance during reception, with power being fed from the antenna terminal I4 to the receiver terminal 3l, showing: the input voltage standing wave ratio (VSWR) in terminal I4; the insertion loss in decibels (db) from terminal I4 to terminal 3l', and the overall directivity or level of antenna power at the transmitter terminal I5 compared to the level of power at the receiver terminal 31, expressed in decibels down (-db) Frequency Mc./Sec. Loss db Dirfgity VSWR (in terminal I5) 1.1

Insertion loss (from terminal I5 to terminal 31) Leakage power 19 microwatts T-R tubes The insertion loss of 2.4 db is the loss customarily encountered with type 1B63 tubes. It is a constant quantity for .the tube, and hence the loss in db of the duplexer decreases with increasing power from the magnetron.

III. The magnetron decoupling was found to be superior to that of conventional duplexers over the band from 8575 to 9660 mc./sec. If an adjustable plunger is putin the arm of the transmitter terminal I5, and a signal is fed from the antenna terminal I4 to the receiver terminal 31, the eiect of variations in the position of the plunger arm will give an indication of magnetron, or transmitter decoupling. In a conventiona1 or single tube duplexer, the plunger will have no effect at the design frequency, but at the edges of the band (8575 to 9660 mc./sec.), sharp resonances as high as 5 db have been observed. These resonances are observed at variations in the power present in the receiver terminal 31. Tests made on the present balanced duplexer show no eiect at the design wavelength, and a maximum variation of 1.55 db at the worst edge of the band. This was a gradual Variation, and not a sharp change. The data is Variation in Terminal 37 Frequency .0999? nuoce-ou cocoon tically any form of directional coupler which couples two substantially parallel line sections through a common wall, so that power entering one of the lines at one end emerges from both lines at the other end. However, other forms may be employed, as those skilled in the art will readily recognize; for example, coupling need not lnecessarily be effected through a common wall. It is accordingly intended that the claims that follow shall not be limited by the details of the illustrated embodiment, but only by the prior art.

In the claims that follow, the term directional coupler shall be understood to meana device consisting of a lrst, or main, electromagnetic -wave transmission line and a second, or auxiliary, electromagnetic wave transmission line, each line having two ends, or terminals, the lines being coupled together at a point, or points, interme- -diate the ends of each, which, when there exist waves travelingr in `both directions in the main line, delivers toV one end-of the auxiliary line a voltage which is largely a function of the amplitude of the wave going in one preferred direction in the main line, and relatively independent of the wave going in the opposite direction inthe main line.

I claimz:

1. A duplexer adapted to bev used with ultrahigh frequency electromagnetic wave energy comprising: `rst and second electromagnetic Wave transmission line sections, each section having a rst end and a second end; means directionally coupling said sections together at each of two spaced-apart regions, each of said coupling means being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said coupling means; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude.

2. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: first and second electromagnetic wave transmission line sections, each section-having a rst end and a second end; first and second directional couplers arranged directionally to couple said sections together at each of two spaced-apart regions, each of said couplers being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said couplers; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of avoltage exceeding a certain magnitude.

3. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: ilrst and second electromagnetic wave transmission line sections, each section having a rst end and a second end; first and second electrically similar directional couplers arranged directionally to couple said sections together at each of two spaced-apart regions, each of said couplers being arranged to couple energy arriving thereat directly from an end of one of said line sectionsinto the other of said line sections propagating substantially exclusively toward the other of said couplers; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude.

4. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: rst and second electromagnetic wave transmission line sections, each section having a rst end and a second end; rst and second quadrature phase change directional couplers arranged directionally to couple said sections together at each of two spaced-apart regions, each of said couplers being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating -substantially exclusively t0- ward the other of said couplers; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude.

5. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: lrst an-d second electromagnetic wave transmission line sections, each section having ,a rst end and a second end; rst andsecond quadrature phase change directional couplers ar'- ranged directionally to coupleA said sections together at each of two spaced-apart regions, each of said couplers being arranged to couple energy arrivingthereat directly from yani-end of one of rsaid linezsections into the other of said lines'ec- 'tions propagating substantially exclusively toward the other of said couplers; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude, said impedance changing means both 'being substantially equidistant from the first of said coupling regions, and substantially equidistant from the second of said coupling regions.

6. A duplexer adapted to be used with ultrahgh frequency electromagnetic wave energy comprising: first and second electromagnetic wave transmission line sections, each section having a rst end and a second end; rst and second hybrid directional couplers arranged directionally to couple said sections together at each of two spaced-apart regions, each of said couplers being :arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said couplers; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude.

7. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: rst and second electromagnetic wave transmission line sections, each section having a rst end and a second end; rst and second symmetrical hybrid directional couplers arranged directionally to couple said sections together at each of two spaced-apart regions, each of said couplers being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said couplers; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude.

8. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energls7 comprising: first and second electrically similar electromagnetic wave transmission line sections,

veach section having a first end and a second end; vmeans directionally coupling said sections together at each of two spaced-apart regions, each of said coupling means being arranged to couple energy arriving thereat directly from an end of -one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said coupling means; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presen-ce in said line of a voltage exceeding a certain magnitude. l

9. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: rst and second electrically similar electromagnetic wave transmission line sections,

each section having a rst end and a second end; vmeans directionally coupling said sections together at each of two spaced-apart regions, said means being electrically symmetrical, each of -said coupling. means being arranged to couple l.energy arriving' thereat directly from an end of one of said line sections into the other'of said line sections propagating substantially exclusively toward the other of said coupling means; and

,means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude.

10. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: first and second electrically similar electromagnetic wave transmission line sections, each section having a first end and a second end; means directionally coupling said sections together at each of two spaced-apart regions, said means being electrically symmetrical, each of said coupling means being arranged to couple energy arriving thereat directly from an end'of one of said line sections into the otherof said line sections propagating substantially exclusively toward the other of said coupling means; and means in each line intermediate said two regions for causingv an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude, said impedance changing means* both being substantially equidistant from the first of said coupling regions, and substantially equidistant from the second of said coupling regions.

11. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: rst and second electrically similar electromagnetic wave transmission line sections, each section having a first end and a second end; electrically similar directional coupler means directionally coupling said sections together at each of two spaced-apart regions, each of said coupling means being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said coupling means; and means in each line intermediate said two regions for causing an abrupt change in the impedance characteristic of the line in response to the presence in said line of a voltage exceeding a certain magnitude.

12. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: rst and second electromagnetic wave transmission line sections, each section having a first end and a second end means directionally coupling said sections together at each of two spaced-apart regions, each of said coupling means being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said coupling means; and a discharge device connected across each section at a third region intermediate said two regions for providing substantially a short circuit across each line at said third region in response to the presence -in said line of a voltage exceeding a predetermined magnitude.

13. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: first and second electromagnetic wave rectangular waveguide sections, each section having a first end and a second end, said sections sharing a common wall in two separate regions spaced apart along the waveguide; means in each of said regions for directionally coupling said sections together at each region through the common wall, each of said coupling means vbeing arranged to couple energy arriving thereat directly from an end of one of said line sections `intothe .other of said line sections propagatins 13 substantially exclusively toward the other of said coupling means; and means in each section intermediate said regions for causing an abrupt change in the impedance characteristic of the section in response to the presence in said section of a voltage exceeding a certain magnitude.

14. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: rst and second electromagnetic wave rectangular waveguide sections, each section having a first end and a second end, said sections sharingr a common wall in two separate regions spaced apart along the waveguides; quadrature phase shift directional coupler means in each of said regions for directionally coupling said sections together at each region through the common wall, each of said coupling means being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said coupling means; and means in each section intermediate sad regions for causing an abrupt change in the impedance characteristic of the section in response to the presence in said section of a voltage exceeding a certain magnitude.

15. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: rst and second electromagnetic wave rectangular waveguide sections, each section having a rst end and a second end, said sections sharing -a common wall in two separate regions spaced apart along the waveguides; quadrature phase shift directional coupler means in each of said regions for directionally coupling said sections together at each region through the common wall, each of said coupling means being arranged to couple energy arriving thereat directly from an end of` one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said coupling means; and means in each section intermediate said regions f or causing an abrupt change in the impedance characteristic of the section in response to the presence in said section of a voltage exceeding a certain magnitude, said impedance changing means both being equidistant from the first of said regions, and equidistant from the second of said regions.

16. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: first and second electromagnetic wave rectangular waveguide sections, each section having a rst end and a second end, said sections sharing a common wall in two separate regions spaced apart along the waveguides; hybrid directional coupler means in each of said regions for directionally coupling said sections together at each region through the common wall, each of said coupling means being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said coupling means and means in each section intermediate said regions for causing an abrupt change in the impedance characteristic of the section in response to the presence in said section of a voltage exceeding a certain magnitude.

17. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: rst and second electromagnetic wave rectangular waveguide sections, each section having a rst end and a second end, said 5 sections sharing a common wall in two separate regions spaced apart along the waveguides; a directional coupler consisting of slots in the common wall in each of said regions, each of said couplers being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of-said couplers; vand means in each section intermediate said regions for causing an abrupt change in the impedance characteristic of the section in response to the presence in said section of a voltage exceeding a certain magnitude.

18. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: irst and second electromagnetic wave rectangular waveguide sections, each section having a rst end and a second end, said sections sharing a common similar wall in two separate regions spaced apart along the waveguides; a directional coupler consisting of slots in the common wall in each of said regions, each ofsaid couplers being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said couplers; and means in each section intermediate said regions for causing an abrupt change in the impedance characteristic of the section in response to the presence in said section of a'voltage exceeding a certain magnitude.

19. A duplexer adapted to be used with ultrahigh frequency electromagnetic wave energy comprising: first and second sections of similar 40 electromagnetic wave waveguide, each section having a rst end and a second end, said sections sharing a common similar wall in two separate regions spaced apart along the waveguides; a directional coupler consisting of slots in the common wall in each of said regions, each of said couplers being arranged to couple energy arriving thereat directly from an end of one of said line sections into the other of said line sections propagating substantially exclusively toward the other of said couplers; and means in each section intermediate said regions for causing an abrupt change in the impedance characteristic of the section in response to the presence in said section of a voltage exceeding a certain magnitude, said impedance changing means both being equidistant from the rst of said regions, and equidistarri', rfgilifri Vlille second of said regions.

` HENRY J. RIBLET.

REFERENCES CITED The following references are of record in the ille of .this patent:l

-----UNrrED"'sTATEs PATENTS 

